STRATIGRAPHIC FRAMEWORK AND FACIES CHANGES IN THE HIRNANTIAN-RHUDDANIAN TRANSITION SAN JUAN, PRECORDILLERA ARGENTINA: PALEOENVIRONMENTAL AND TECTOSEDIMENTARY SIGNIFICANCE.

JESSICA GOMEZ; PERALTA SILVIO; BERESI MATILDE

Chubut

Congreso; XXI CONGRESO GEOLÓGICO ARGENTINO; 2022

The Hirnantian-Rhuddanian transition is registered in the Central Precordillera, at the basal part of the La Chilca Formation (Late Hirnantian-Early Wenlock) and, in the Eastern Precordillera at the upper part of the Don Braulio Formation (Late Hirnantian-Llandovery). La Chilca Formation is recognized from the Jáchal River area to the north, up to the north of the San Juan River and consists of a storm-dominated thickening-coarsening upward siliciclastic succession bounded by transgressive surfaces. This unit overlies marine deposits from the Late Ordovician age, in the northern sector of the basin (Los Azules, Las Aguaditas and Don Braulio formations), and of Early Ordovician age (San Juan Formation) in the southern sector of the basin. At the top is paraconformably overlain by the Los Espejos Formation (Middle-Upper Silurian). The Don Braulio Formation crops out on the eastern flank of the Villicum range; it is made up of a siliciclastic shallow-water succession, with Hirnantian glacimarine diamictites at the base, and ferruginous oolitic bars of Llandovery age at the upper part. In this work, a brief stratigraphic and lithofacial analysis of the Ordovician-Silurian transition is carried out in the Talacasto, Don Braulio Creek and the Cerro La Chilca areas, in order to provide information related to the paleogeography of the Early Paleozoic basin of Precordillera.At the Talacasto locality, the Los Baños, Ancha creek and Poblete Norte sections were studied (Fig. 1); in the first one, La Chilca Formation starts with a cherty pebbly basal conglomerate, conformably overlain by laminated pelites of the Salto Macho Member, bearing graptolites of the P. persculptus Zone (Hirnantian age), and P. acuminatus and A. atavus Zones of Rhuddanian age (Cuerda et al., 1988). In the Ancha Creek section, the basal conglomerate underlies in sharp contact, a ferruginous sandstones bed with scattered clasts of chert, overlain by a ferro-phosphate oolitic level and laminated pelites of the Salto Macho Member. In the Poblete Norte section, this formation overlies in erosive contact, fossiliferous limestone of the San Juan Formation. Its basal level is characterized by a carbonatic sandstones bed passing into ferruginous sandstones with ferro-phosphatic and scattered clasts of chert. This succession is overlain by ferruginous sandstones with oolites, covered by a ferro-phosphate oolitic level in sharp contact with the Cuarcitas Azules Member. In this section, it is noteworthy the absence of the basal conglomerates and pelites with Hirnantian-Llandoverian fauna of the Salto Macho Member. In the Cerro La Chilca section, La Chilca Formation overlies, in erosional contact, fossiliferous deposits of the Don Braulio Formation (Astini and Benedetto, 1992), and its succession starts with a ferruginous sandstone level including ferro-phosphate nodules and chert clasts, which in part are interbedded with greenish pelites and brown sandstones, which gradually pass into pelites and sandstones of the Cuarcitas Azules Member. It is noted that the basal conglomerate, oolitic levels and pelites of the Salto Macho Member have not been observed in this section. In the Villicum range, the Don Braulio Formation is characterized by a shallow-water succession, starting with a basal glaci-marine diamictite level, covered by sandstones and grey mudstones, bearing trilobites and brachiopods of the Hirnantia Fauna, capped by a fine-grained level which yield P. persculptus (Peralta and Baldis, 1990). Upward, ocher mudstone succession occurs with evidences of up-welling processes, such as bioturbation, massive fabric and abundance of organic matter, phosphate and iron. This succession, on top is in sharp contact covered by oolitic bars, with ferruginous sandstones levels including scattered clasts of chert and ooids, interbedded with rich-graptolite shales indicating the A. atavus Zone, and palinomorphs of the early Llandovery (Peralta, 1985). The Ordovician-Silurian boundary could be placed into the ocher mudstone succession, but the lack of diagnostic fossils record, prevents knowing of the right position of the boundary.In the Ordovician, tectonic extensional processes would have caused the northward tilting and later subsidence of the Early Paleozoic basin, followed by the Hirnantian-Rhuddanian glacial and post-glacial events. In this scenario, the uplift of the Tambolar high in the San Juan River area, and consequent northward and westward basin deepening (Peralta, 2013), would have favored shallow facies development towards the structural high, but also the facies change of La Chilca Formation towards the south and west. In the Eastern Precordillera, glaciomarine diamictites and subsequent development of a siliciclastic platform with Hirnantia Fauna are the results of the glacial and post-glacial events occurred in western Gondwana during the Hirnantian. This structural framework could have conditioned deposition of the basal conglomerate of La Chilca Formation, in the Cerro La Chilca, Poblete Norte and Don Braulio sections, but on the other side, could have favored development of ferruginous sandstone levels with chert clasts in the Cerro La Chilca and Poblete Norte sections. At the Ancha creek and Baños de Talacasto sections, the ferruginous and ferro-phosphate oolitic deposits are covered by transgressive pelites bearing the biozone of P. persculptus (Cuerda et al, 1988), meanwhile in the Don Braulio section, regressive deposits are associated with graptolite-bearing pelites from the A. atavus zone. Between these sections there is a lithofacies change in the oolitic levels with reworking evidence, but also in the architecture of deposit and a significant presence of iron to the east. The latitudinal position of Precordillera in the Upper Ordovician, between 30º and 45º S (Torsvik and Cocks, 2009), frames it in temperate to cold climate which would have controlled the sedimentation and iron supply in the Silurian basin (Astini, 1992), favoring the development of upwelling type deposits in the Ocher Mudstones Member, rich in phosphates and iron.